Hadrosaur diet
, showing its multiple rows of leaf-shaped teeth. The worn, chewing surface of the teeth is towards the top.]] Hadrosaurids, the duck-billed dinosaur species commonly referred to as hadrosaurs, were large terrestrial herbivores. The '''diet of hadrosaurid dinosaurs' remains a subject of debate among paleontologists, especially regarding whether hadrosaurids were grazers who fed on vegetation close to the ground, or browsers who ate higher-growing leaves and twigs. Preserved stomach content findings have indicated they may have been browsers, whereas other studies into jaw movements indicate they may have been grazers. The mouth of a hadrosaur had hundreds of tiny teeth packed into dental batteries. These teeth were continually replaced with new teeth. Hadrosaur beaks were used to cut food, either by stripping off leaves or by cropping. It is believed hadrosaurs had cheeks in order to keep food in the mouth. Researchers have long believed their unusual mouth mechanics may have played a role in their evolutionary success. However, because they lack the complex flexible lower jaw joint of today's mammals, it has been difficult for scientists to determine exactly how the hadrosaurs broke down their food and ate. Without this understanding, it had been impossible to form a complete understanding of the Late Cretaceous ecosystems and how they were affected during the Cretaceous–Tertiary extinction event 65 million years ago. It has also remained unclear exactly what hadrosaurids ate. In particular, it had never been definitively proven whether hadrosaurs were grazers who ate vegetation close to the ground, like modern-day sheep or cows, or whether the dinosaurs were browsers who ate higher-growing leaves and twigs, like today's deer or giraffes. A 2008-2009 study by University of Leicester researchers analyzed hundreds of microscopic scratches on the teeth of a fossilized Edmontosaurus jaw and determined hadrosaurs had a unique way of eating unlike any creature living today. In contrast to a flexible lower jaw joint prevalent in today's mammals, a hadrosaur had a unique hinge between the upper jaws and the rest of its skull. The team found the dinosaur's upper jaws pushed outwards and sideways while chewing, as the lower jaw slid against the upper teeth. Coprolites (fossilized droppings) of some Late Cretaceous hadrosaurs show that the animals sometimes deliberately ate rotting wood. Wood itself is not nutritious, but decomposing wood would have contained fungi, decomposed wood material and detritus-eating invertebrates, all of which would have been nutritious. Early history of research The first hadrosaur finds did not include much skull material. Hadrosaur teeth have been known since the 1850s (Joseph Leidy's Trachodon), and a few fragments of teeth and jaws were among the bones named Hadrosaurus by Leidy in 1858. (The skeletal mount made for Hadrosaurus by Benjamin Waterhouse Hawkins included a speculative iguana-like skull) Leidy had enough skeletal material to make other inferences about the paleobiology of hadrosaurs, though. Of particular importance was the unequal lengths of the forelimbs and hindlimbs. He interpreted his new animal as a kangaroo-like animal that browsed along rivers, using its forelimbs to manipulate branches. His vague inference of amphibious habits would later be expanded upon by Edward Drinker Cope, who contributed the mistaken conclusion that hadrosaur teeth and jaws were weak and suitable only for eating soft water plants. Cope described the next piece of the puzzle in 1874: a more complete jaw fragment in 1874 he named Cionodon arctatus, which revealed for the first time the complex hadrosaur tooth battery. However, the first essentially complete hadrosaur skull was not described until 1883. It was part of a skeleton (the first essentially complete hadrosaur skeleton as well) collected in 1882 by Dr. J. L. Wortman and R. S. Hill for Cope. Described as a specimen of Diclonius mirabilis, it is now known as the type specimen of Anatotitan. Cope immediately drew attention to the anterior part of the skull, which was drawn out, long, and wide. He compared it to that of a goose in side view, and to a short-billed spoonbill in top view. Additionally, he noted the presence of what he interpreted as the remnants of a dermal structure surrounding the beak. Significantly, Cope regarded his Diclonius as an amphibious animal consuming soft water vegetation. His reasoning was that the teeth of the lower jaw were weakly connected to the bone and liable to break off if used to consume terrestrial food, and he described the beak as weak as well. Unfortunately for Cope, aside from misidentifying several of the bones of the skull, by chance the lower jaws he was studying were missing the walls supporting the teeth from the inside; the teeth were actually well-supported.Lull and Wright, Hadrosaurian Dinosaurs of North America, pp. 43. While Cope anticipated publishing a full report with illustrations, he never did so, and instead the first accurate illustrated description of a hadrosaur skull and skeleton would be produced by his great rival, Othniel Charles Marsh. While Marsh corrected several anatomical errors, he retained Cope's postulated diet of soft plants. The description of hadrosaurs as amphibious eaters of aquatic plants became so ingrained that when the first possible case of hadrosaur gut contents was described in 1922 and found to be made up of terrestrial plants, the author made a point of noting that the specimen only established that hadrosaurs could eat land plants as well as water plants. 1984 hadrosaurid chewing hypothesis In 1984, David B. Weishampel proposed a new hypothesis on how hadrosaurids fed. His study of the sutures between bones in fossil skulls concluded that ornithopods, a group of bird-hipped dinosaurs that includes hadrosaurids, had flexible upper jaws and that when the lower jaw clamped shut, pressure would spread outward from both sides of the upper jaw. The upper teeth would grind against the lower teeth like rasps, trapping the plants and grinding them up. The theory remained largely unproven until the study by Purnell, Williams and Barrett, which Science magazine called, "The strongest independent evidence yet for this unique jaw motion". However, in 2008, a group of American and Canadian researchers, led by vertebrate paleobiologist Natalia Rybczynski, replicated Weishampel's proposed chewing motion using a computerized three-dimensional animation model. Rybczynski et al. believe Weishampel's model may not be viable, and plan to test other hypotheses. 2008 perserved stomach content findings by Tweet In 2008, when a team led by University of Colorado at Boulder graduate student Justin S. Tweet found found a homogeneous accumulation of millimeter-scale leaf fragments in the gut region of a well-preserved partially-grown Brachylophosaurus. As a result of that finding, Tweet concluded in September 2008 that the animal was likely a browser, not a grazer. 2008–2009 hadrosaur chewing study by Williams et al. '' (pictured), and concluded that duck-billed dinosaurs likely grazed on vegetation close to the ground and had a way of chewing unlike any modern animal.]] A study into exactly how a hadrosaur broke down and ate its food was conducted by Vince Williams, a graduate student at the University of Leicester; Paul Barrett, a paleontologist with London's Natural History Museum; and Mark Purnell, a British paleontologist from the geology department of the University of Leicester. The three men employed a new approach to analyze the feeding mechanisms of dinosaurs, and thus help understand their place in the prehistoric ecosystems. Chewing on solid food always leaves tiny scratches on the teeth's surfaces. The trio believed that by looking at the size and orientation of those markings on hadrosaurid teeth, they would be able to learn about the movements of their jaws. Purnell said although he believed this form of study could help determine how and what the hadrosaur ate, he said no previous studies had ever employed this type of analysis. Wiliams, Barrett, and Purnell conducted their study using the jaws of an Edmontosaurus, a crestless hadrosaurid that lived between 68 and 65 million years ago in what is now the United States and Canada. The specific Edmontosaurus jaw used in this study was collected from Late Cretaceous rocks found in the United States. The individual teeth on the jaw contained multiple hundreds of microscopic scratches, which had been preserved intact during fossilization. The researchers carefully cleaned the jaws, molded them and coated them with gold to make a detailed replica of the tooth surface. Then they used a scanning electron microscope to give high-power magnification of the scratches for study, and conducted a three-dimensional statistical analysis of the direction of the scratches. Findings The study found that the hadrosaur chewed using a method completely different than any creature living today, and utilized a type of jaw that is now extinct. The study found the Edmontosaurus jaw had four different sets of parallel scratches running in different directions. Purnell concluded each set of scratches related to a specific jaw movement. This revealed the movement of hadrosaurs was complex and employed movement in several different directions, including up-and-down, front-to-back and sideways movements. The trio concluded that in contrast to the flexible lower jaw joint prevalent in modern mammals, the hadrosaur had a hinge between its upper jaws and the rest of its skull. According to the study, the hadrosaur would push its upper jaws outwards and sideways, while the lower teeth slid against the upper teeth. As the tooth surfaces slid sideways across each other, the food would be ground and shredded before consumption. Purnell said the style of eating, "was not a scissor-like movement; it seems that these dinosaurs invented their own way of chewing." Although the upper-jaw teeth hinged outward when the hadrosaur ate, Purnell said it was likely the dinosaur could still chew with its mouth closed. While the outward flexure of the upper jaws might have been visible, Purnell said the chewing was likely concealed by the hadrosaur's cheeks and probably looked "quite subtle". , Equisetum telmateia]] The study also made conclusions about what hadrosaurids ate, although Purnell cautioned the conclusions about the hadrosaur's diet were "a little less secure than the very good evidence we have for the motions of the teeth relative to each other." The scratches found on each individual tooth were so equal that measuring an area of just one square millimeter was enough to sample the whole jaw. The team concluded the evenness of the scratches suggested the hadrosaur used the same series of jaw motions over and over again. As a result, the study determined that the hadrosaur diet was probably made of leaves and lacked the bulkier items such as twigs or stems, which might have required a different chewing method and created different wear patterns. The lack of pit marks on the teeth also upheld these conclusions, and suggested the hadrosaurs likely grazed on low-lying vegetation that lacked pits, rather than browsing on higher-growing vegetation with twigs. The scratches also indicated the hadrosaur's food contained either small particles of grit, which was normal for vegetation cropped closed to the group, or that it contained microscopic granules of silica, which is common in grass. Grasses had evolved by the Late Cretaceous period, but were not particularly common, so the study concluded it probably did not play a major component in the hadrosaur's diet. Instead, they believed horsetails, a common plant at the time containing the above characteristics, was probably an important food for the dinosaur. Publication The results of the study were published online on June 30, 2009, in The Proceedings of the National Academy of Sciences, the official journal of the United States National Academy of Sciences. The study was published under the title, "Quantitative analysis of dental microwear in hadrosaurid dinosaurs, and the implications for hypotheses of jaw mechanics and feeding". It was the first quantitative analysis of tooth microwear in dinosaurs. Purnell said the technique employed in the study was equally important as the findings themselves, and that the study proved analyzing microscopic scratch marks on teeth can provide reliable information about an animal's diet and chewing mechanism. Purnell said this method could be used to study other areas of scientific research, including the dietary habits of other long-vanished species including dinosaurs, extinct groups of fish or very early mammals. Purnell said the findings were further significant not only for the basic understanding of how hadrosaurids ate, but also because a lack of such understanding from those dinosaurs represented a "big gap in our knowledge" of the ecosystem of the late Cretaceous. Because hadrosaurs were the dominant terrestrial herbivores of that time, they played a major role in structure the ecosystem of the Late Cretaceous period. Purnell said, "The more we understand the ecosystems of the past, and how they were affected by global events like climate change, the better we can understand how changes now are going to pan out in the future." Lawrence Witmer, a palentologist with Ohio University College of Osteopathic Medicine in Athens, called the study, "One of the best microwear papers I've seen", although he said he was not yet convinced the hadrosaurid upper jaw could flex. Comparison to other cases The scratch patterns from the study confirm a theory about hadrosaur chewing first proposed about 25 years ago, according to the researchers. The hypothesis that hadrosaurs were likely grazers rather than browsers appears to contradicts previous findings from preserved stomach contents found in the fossilized guts in previous hadrosaurs studies. The most recent such finding before the publication of the Purnell study was conducted in 2008, when a team led by University of Colorado at Boulder graduate student Justin S. Tweet found found a homogeneous accumulation of millimeter-scale leaf fragments in the gut region of a well-preserved partially-grown Brachylophosaurus. As a result of that finding, Tweet concluded in September 2008 that the animal was likely a browser, not a grazer. In response to such findings, Purnell said preserved stomach contents are questionable because they do not necessarily represent the usual diet of the animal.This information comes from the aforementioned Alan Boyle source from June 29, 2009. However, this specific information is not included in the body of the article, but rather a response by Boyle to comments in the article. Since the comments were written by Boyle himself, and since they cite information he received specifically from Purnell, they are as legitimate a source of information as the article itself. Alan Boyle, a journalist and MSNBC science editor who reported on Purnell's findings, said of the apparent contradictions between Purnell's study and previous stomach content findings are subject to debate, but do not necessarily render Purnell's study irrelevant or incorrect. Specifically, Boyle said, "the claims about grazing vs. browsing are certainly not conclusive (but) the researcher's surmise is that they were more likely to graze". Purnell's hypothesis of hadrosaurids as grazers who ate vegetation close to the ground, rather than browsing higher-growing leaves and twigs, would also contradict the portrayal of hadrosaurs in Jurassic Park, the 1990 science fiction novel by Michael Crichton. Diet Feeding adaptations of Edmontosaurus As a hadrosaurid, Edmontosaurus was a large terrestrial herbivore. Its teeth were continually replaced and packed into dental batteries that contained hundreds of teeth, only a relative handful of which were in use at any time. It used its broad beak to cut loose food, perhaps by cropping, or by closing the jaws in a clamshell-like manner over twigs and branches and then stripping off the more nutritious leaves and shoots. Because the tooth rows are deeply indented from the outside of the jaws, and because of other anatomical details, it is inferred that Edmontosaurus and most other ornithischians had cheek-like structures, muscular or non-muscular. The function of the cheeks was to retain food in the mouth. Fastovsky, D.E., and Smith, J.B. (2004). "Dinosaur paleoecology." The Dinosauria. pp. 614–626. The animal's feeding range would have been from ground level to around above. Before the 1960s and 1970s, the prevailing interpretation of hadrosaurids like Edmontosaurus was that they were aquatic and fed on aquatic plants. An example of this is William Morris's 1970 interpretation of an edmontosaur skull with nonbony beak remnants. He proposed that the animal had a diet much like that of some modern ducks, filtering plants and aquatic invertebrates like mollusks and crustaceans from the water and discharging water via V-shaped furrows along the inner face of the upper beak. This interpretation of the beak has been rejected, as the furrows and ridges are more like those of herbivorous turtle beaks than the flexible structures seen in filter-feeding birds. ]] The prevailing model of how hadrosaurids fed was put forward in 1984 by David B. Weishampel. He proposed that the structure of the skull permitted motion between bones that led to backward and forward motion of the lower jaw, and outward bowing of the tooth-bearing bones of the upper jaw when the mouth was closed. The teeth of the upper jaw would grind against the teeth of the lower jaw like rasps, processing plant material trapped between them. Such a motion would parallel the effects of mastication in mammals, although accomplishing the effects in a completely different way. An important piece of evidence for Weishampel's model is the orientation of scratches on the teeth, showing the direction of jaw action. Other movements could produce similar scratches, though, such as movement of the bones of the two halves of the lower jaw. Such potential movements have yet to be scrutinized under present techniques. Weishampel developed his model with the aid of a computer simulation. Natalia Rybczynski and colleagues have updated this work with a much more sophisticated three-dimensional animation model, scanning a skull of E. regalis with lasers. They were able to replicate the proposed motion with their model, although they found that additional secondary movements between other bones were required, with maximum separations of between some bones during the chewing cycle. Rybczynski and colleagues were not convinced that the Weishampel model is viable, but noted that they have several improvements to implement to their animation. Planned improvements include incorporating soft tissue and tooth wear marks and scratches, which should better constrain movements. They note that there are several other hypotheses to test as well. Further work by Casey Holliday and Lawrence Witmer found that ornithopods like Edmontosaurus lacked the types of skull joints seen in those modern animals that are known to have kinetic skulls (skulls that permit motion between their constituent bones), such as squamates and birds. They proposed that joints that had been interpreted as permitting movement in dinosaur skulls were actually cartilaginous growth zones. Reports of gastroliths, or stomach stones, in the hadrosaurid Claosaurus are actually based on a probable double misidentification. First, the specimen is actually of Edmontosaurus annectens. Barnum Brown, who discovered the specimen in 1900, referred to it as Claosaurus because E. annectens was thought to be a species of Claosaurus at the time. Additionally, it is more likely that the supposed gastroliths represent gravel washed in during burial. Gut contents " at the American Museum of Natural History, but were never described.]] Both of the "mummy" specimens collected by the Sternbergs were reported to have had possible gut contents. Charles H. Sternberg reported the presence of carbonized gut contents in the American Museum of Natural History specimen, but this material has not been described. The plant remains in the Senckenberg Museum specimen have been described, but have proven difficult to interpret. The plants found in the carcass included needles of the conifer Cunninghamites elegans, twigs from conifer and broadleaf trees, and numerous small seeds or fruits. Upon their description in 1922, they were the subject of a debate in the German-language journal Paläontologische Zeitschrift. Kräusel, who described the material, interpreted it as the gut contents of the animal, while Abel could not rule out that the plants had been washed into the carcass after death. At the time, hadrosaurids were thought to have been aquatic animals, and Kräusel made a point of stating that the specimen did not rule out hadrosaurids eating water plants. The discovery of possible gut contents made little impact in English-speaking circles, except for another brief mention of the aquatic-terrestrial dichotomy, until it was brought up by John Ostrom in the course of an article reassessing the old interpretation of hadrosaurids as water-bound. Instead of trying to adapt the discovery to the aquatic model, he used it as a line of evidence that hadrosaurids were terrestrial herbivores. While his interpretation of hadrosaurids as terrestrial animals has been generally accepted, the Senckenberg plant fossils remain equivocal. Kenneth Carpenter has suggested that they may actually represent the gut contents of a starving animal, instead of a typical diet. Other authors have noted that because the plant fossils were removed from their original context in the specimen and were heavily prepared, it is no longer possible to follow up on the original work, leaving open the possibility that the plants were washed-in debris. Cope's description promoted hadrosaurids as amphibious, contributing to this long-time image. His reasoning was that the teeth of the lower jaw were weakly connected to the bone and liable to break off if used to consume terrestrial food, and he described the beak as weak as well. Unfortunately for Cope, aside from misidentifying several of the bones of the skull, by chance the lower jaws he was studying were missing the walls supporting the teeth from the inside; the teeth were actually well-supported.Lull and Wright, Hadrosaurian Dinosaurs of North America, pp. 43. This specimen, AMNH 5730, was purchased by the American Museum of Natural History in 1899.Lull and Wright, Hadrosaurian Dinosaurs of North America, pp. 157-159. Cope intended to describe the skeleton as well as the skull, but his promised paper never appeared. Notes and references External links * [http://www.pnas.org/content/early/2009/06/26/0812631106 Downloadable version of 2008-2009 Purnell et al study at The Proceedings of the National Academy of Sciences (official)] * Press release about the 2008-2009 Purnell et al study Category:Cretaceous Category:Hadrosaurs Category:Cretaceous Category:Hadrosaurs